This invention relates to drilling and cementing boreholes.
The drilling of boreholes is generally carried out using a rotary drilling process. The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill string is hollow and sections are added to the drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these necessities are accomplished by the circulation of a fluid down through the drill string and up to the surface between the drill string and the wall of the borehole.
Once the borehole has been drilled to the desired depth, it may be desirable to isolate the separate areas, zones or formations transversed by the borehole. For extraction of fluids from formations, and for protection against borehole collapse, one or more conduits (casings) must be inserted into the borehole extending from the surface downward. Thereafter, liners may be hung inside the casings. A liner is any string of casing having its top below the surface of the ground. Liners are used in deep wells to case off an open hole section and thus eliminate a full string of casing.
At this point it becomes necessary to fill the annulus between the casing and the borehole wall or between the liner and casing with a material which will seal the annulus to inhibit communication between various formations penetrated by the wellbore and which will provide structural support for the casing or liner. This is commonly referred to as primary cementing.
Generally, the borehole, into which the casing or liner to be cemented is installed, is filled with drilling mud. Therein lie several problems. Conventional Portland cement and conventional drilling muds are incompatible. Thus, as the cement is forced down the casing or liner and up into the annulus it is commingled with the drilling mud at any interface between the mud and the cement. The resulting mixture generally thickens or becomes a gel and does not set up into a strong cement. In addition the gel strength and viscosity become uncontrollable and the mixture may become too viscous to pump. Alternatively, the mixture may get thinner and cause solids to settle down in the annulus where they may bridge and restrict passage of the cement slurry. In either event, the situation is unsatisfactory. Furthermore, the used drilling fluid must be disposed of which adds an additional procedure to the drilling operation. Also, in the case of oil base muds there are increased environmental concerns with drilling fluid disposal.
Hale et al, U.S. Pat. No. 5,058,679 (Oct. 22, 1991) broadly disclose mixing blast furnace slag with drilling mud to produce a cement, thus avoiding the necessity for disposing of the drilling fluid. As the patent discloses, the drilling fluid is compatible with the blast furnace slag and good cement results. However, drilling fluid ingredients in the resulting cementitious slurry can still interfere with Portland cement if a Portland cement tail is desired following a blast furnace slag lead. Cowan, U.S. Pat. No. 5,016,711 (May 21, 1991) broadly discloses improved cement adhesion through the use of a surfactant. Parceveaux et al., U.S. Pat. No. 5,101,902 (Apr. 7, 1992) broadly describes spacers between mud and cement.